This invention is related in general to the field of electrical and electronic circuits, and more particularly, to a supply voltage reference circuit.
VDD reference circuits supply a reference voltage to other circuits in a system. A VDD reference circuit is sometimes required to be overvoltage tolerant and have minimum Ioff current. Overvoltage is a condition that occurs when the output pin of the VDD reference circuit is pulled a threshold voltage (VDD) above the VDD. Ioff refers to the maximum leakage current that flows into or out of the input or output nodes when the input or output is forced to a given DC voltage when VDD is zero. However, conventional VDD reference circuits, although satisfying these requirements, suffer from the disadvantage of always supplying a smaller VDD voltage during normal circuit operations. The decreased VDD is caused by the large voltage drop across the blocking diode. As the voltage level of VDD becomes smaller, the voltage drop across the blocking diode becomes a more significant factor leading to reduced speed of circuit devices. Conventional VDD reference circuits are also disadvantageous due to the use of a schottky diode that is typically coupled in parallel with the blocking diode. Schottky diodes are undesirable because they are typically leaky by nature.
It has been recognized that it is desirable to provide a supply reference voltage circuit that satisfies Ioff and overvoltage tolerance requirements, as well as bypasses the current blocking diode during normal operations.
In one aspect of the invention, a supply reference voltage circuit is coupled to an output node, a supply voltage node and a supply reference voltage node and is operable to connect the output node to the supply reference voltage node and prevent current flow through an output device coupled to the output node in response to sensing a low voltage level at the supply voltage node and a non-zero voltage at the output node. The circuit is further operable to connect the supply reference voltage node to the supply voltage node in response to the voltage at the output node being a threshold voltage above the voltage at the supply voltage node. The circuit is further operable to bypass a blocking diode in response to sensing a high voltage level at the supply voltage node.
In another aspect of the invention, a supply reference voltage circuit generates a stable supply reference voltage from a supply voltage. The circuit includes a blocking diode coupled between the supply voltage and the supply reference voltage, and an output node. The circuit further includes a first circuit coupled to the output node, the supply voltage and the supply reference voltage and operable to prevent current flow through an output device coupled to the output node in response to sensing a low supply voltage level and a non-zero output voltage level. The first circuit further operable to connect the output node to the supply reference voltage in response to the voltage at the output node being a threshold voltage above the supply voltage level. The circuit further includes a second circuit coupled to the supply voltage and the supply reference voltage operable to connect the supply reference voltage to the supply voltage and bypass the blocking diode in response to sensing a high supply voltage level.
In yet another aspect of the invention, a method of providing a supply reference voltage includes the steps of turning off an output transistor in response to sensing a non-zero voltage level at the output node and a zero supply voltage level, connecting the supply reference voltage to the voltage at an output node coupled to the output transistor in response to sensing the voltage at the output node being a threshold voltage above the supply voltage level, and also providing a path from a supply voltage to the supply reference voltage and thereby bypassing a current blocking diode coupled between the supply voltage and the supply reference voltage.
One technical advantage of the invention is the reduction of Ioff, satisfying overvoltage tolerance requirements, as well as bypassing the blocking diode to provide the full potential of VDD.